[NTLUG:Discuss] "Back up dns server"?

[Chris Cox]

m m wrote:
....
> My questions is:
>  
> How can I have a "back up" (web) server (at location A city) take over the 
> job when the primary web server (at location B city) can not reachable
> (because of firewall, power down ...) by using same domain name.

This is VERY hard to do.  Especially if it's a "job", that is an
inflight session with state information.  Easier if you just want
a different non-related web server to answer back and the site
is 100% static content, or even dynamic content where you have
some kind of mirroring happening to keep them insync (and don't
mind if they are a tad out of sync if one goes down).

The problem with trying to handle this with DNS is caching.
If everyone is caching www as 1.2.3.4 as your web server address, nobody
will be able to reach you if www changes to 4.3.2.1 since everyone
thinks your at 1.2.3.4.  You could set a really low value of time to
live on your DNS for other to cache, but it's generally not nice
to have extremely low TTLs.... defeats the scalability of DNS.

If you just need load balancing or failover across sites, you
will have some kind of front end that redirects appropriately
(btw, the frontend can NEVER go down.... that's just life).
With a frontend, even clustering and session management might
be possible.  Somebody already mentioned one kind of frontend...
a "proxy".  But likewise, that "proxy" can't go down, or you
still have the original problem.